# Phil 89: Qualitative Duplicates, Numerical Identity, Change, and Essential Properties

1. Recall we distinguished between being qualitatively the same or identical (being exactly alike/similar) and being numerically identical (one and the same object). Some writers call the second notion “strict identity”; others call it “quantitative identity.”

We had an example where a mother (call her Tamar) has two children Abe and Alicia. For Christmas, she gets them each a new red bike. She makes sure to get two bikes of the same design and color, else the kids would get upset. So now: Abe and Alicia have the same bike, and they also have the same mother. But they have two bikes, and only one mother. Their bikes are the same in the sense of being qualitatively identical. They are very similar to each other. There is still one bike for one child, and another bike for the other child. By contrast, they have the same mother in the sense of Abe’s mother being numerically identical to Alicia’s mother. Unlike the bikes, there is just one mother, Tamar, whom they have to share. One and the same person is mother to both of them.

This isn’t a difference about bikes and mothers. In other sorts of cases, you may talk about “the same bike” where you also mean one and the same bike, not just another duplicate bike.

2. At the microscopic level, two bikes probably have some tiny differences, even if they came off the same production line. But it will be convenient for some of our discussions to ignore this, or pretend the bikes are exactly similar, down to the last detail. They are perfect copies of each other. In such cases, we’ll say the bikes are perfect duplicates. They share all of their properties like size, shape, weight, color, and so on. Philosophers call these intrinsic properties. The bikes may differ in other properties, like their location, who owns them, and so on. We call these extrinsic or relational properties. We’ll be considering the contrast between these properties more later this week. For now, it’s enough to note that being perfect duplicates means you share all your intrinsic properties, but might not share all your relational properties. Two pieces of chalk fresh out of the box may be (more or less) duplicates, but the one piece of chalk will be in my left hand, and the other won’t be.

Do you know any genetic twins? They can have very different tastes and opinions. If they have different careers, and went to different schools, then we can expect them to know different things. On a given Monday afternoon, one twin might be thinking about a math problem, and the other one thinking about dinner. What properties they have isn’t just a function of their genes. Their environments also play a role.

If we ever manage to grow a clone of you, the relationship between you and your clone would be like the relation between genetic twins. (Except that in the clone case, the “twin” was born much later.)

So twins and clones generally won’t be perfect duplicates, in the sense we’ll want to consider. We’ll have to imagine more fantastical set-ups. When we have two duplicates, if one of them is digesting a ham sandwich, then so too is the other. If the one duplicate has a chipped tooth, or a broken arm, or a migraine headache, then so too does the other duplicate. At a given moment, their brains would be perfect Xerox copies of each other.

When objects or people are perfect duplicates at some moment t, that doesn’t mean they will remain duplicates. Even if our universe is perfectly deterministic (which isn’t clear), things that start out as duplicates can encounter different stimuli, because they’re in different locations. So they can gradually end up diverging. When we talk about perfect duplicates, we’ll be focusing on times before they start to diverge.

(Them being duplicates at t doesn’t mean they were always duplicates before t, either. But many times we’ll be thinking about cases where two objects or people were duplicates from the start of their existence up until time t.)

3. As we’ve said before, it’s natural to think that objects can change some of their properties over time. Even a fly frozen in amber can change its location; flies not frozen can also change their shape as they flap their wings; and arguably flies can also lose some of their body parts, but continue to exist. More controversially but still plausibly, some philosophers will argue that the same object that used to be a living fly can now continue to exist as a dead fly. (In the funny philosopher’s usage I mentioned before, the fly has “survived” the change from being alive to being dead, in that it’s one thing that underwent that change, and continued to exist through it; despite no longer being alive.)

4. If we zapped the fly with a laser beam and turned it into a cloud of vapor, then arguably it wouldn’t exist anymore. So there are some changes where, if they happen, the fly no longer exists. Which changes can objects continue to exist through, and which changes destroy them? Which properties can objects lose or gain, and numerically the same object still be around?

These questions are strongly disputed; and for different kinds of objects different kinds of answers may be needed. In the examples I just gave, I proposed that the fly could change its location, shape, lose some of its body parts, and even continue to exist after it died. But when vaporised by a laser, nothing remains anymore that we can say “is still the fly.” (The chemicals that made the fly up will still exist, now in vaporised form.)

Here’s the language philosophers use. They say that the essential properties of an object are the properties it’s impossible for the object to exist without. They’ll also describe these properties as being part of the object’s nature or essence. These need not be the same as the object’s important or interesting or valuable properties. Consider a clock. Many of its important properties, such as whether it works correctly, may be such that the clock can lose them while still being (numerically) the same clock. Since the clock is still around — it used to work correctly but now doesn’t — working correctly isn’t a property it’s impossible for the clock to lose or exist without. That’s not an essential property of the clock.

Which properties would be essential to the clock? Perhaps: being a clock, being solid (instead of a vapor or gas), being composed of more than one molecule. For some of the clock’s properties, it may be difficult to say whether those properties are essential or not.

Similarly, an athlete may value her dedication and agility. These properties are important to her. They may be central to how she thinks about herself. But it’d be possible for her to lose her dedication and agility while still being the same person. She could still continue to exist. She might complain, “I used to be more agile; I used to compete in events so-and-so.” She’s still numerically the same person who formerly did those things. Now, these may be qualitative changes she doesn’t like, and they may fundamentally change her personality and outlook on life. But they aren’t essential properties in the philosopher’s sense.

What properties are essential to the athlete, or to you? What these are is controversial and we’re not yet in a position to propose a list. For now, it’s enough that we understand what it means to say some property is essential to you. It means: were that property to be gone, you would no longer exist. Another person might, or might not, exist in your place. But you could not continue anymore to exist, without having that property. That’s what it means for a property to be essential.

Your “nature” in this sense of your essential properties isn’t the same as talking about a “human nature” that merely disposes you a certain way. Nor is it the same as talking about your “fate” (if have a fate).

Those properties which are not essential, philosophers call accidental properties. This is a specific technical sense of “accidental,” which diverges from the ordinary folk meaning. Suppose our athlete eats an excellent careful diet and exercises for hours every day. In the philosopher’s sense, her being in good shape is still an accidental property — she would still be the same person if she stopped being in good shape. But of course, it’s not a coincidence that she’s in good shape. It’s not an “accident” in the ordinary folk meaning of that word.

I’ll try to avoid this philosopher’s use of “accidental” and will just say “non-essential” instead.

5. Sometimes philosophers talk about identity conditions. Identity conditions for a table or tree (in general) are what it takes for something to count as a table, or count as a tree. In some cases, as with trees, plausibly this will include what the thing is made of. To be a tree, you have to be made of wood. (That’s not all it takes. My shelf is made of wood, and it isn’t a tree. But being made of wood is part of what it takes to be a tree.) In other cases, as with tables, plausibly there is more flexibility in what the thing is made of. Tables can be made of wood, of metal, plastic, glass, ice… To be a table, it seems to be more important why a thing was made, and/or how the thing is now being used.

These are all conditions for being a tree or a table. Philosophers are also interested in conditions for being one and the same particular tree or table, and these are also called “identity conditions.” Some of you wrote about cases where it’s unclear or disputable how many persons are occuping a single body at the same time. An account of what it takes to be one person (or one table, or tree, or bicycle, or ship), rather than two, is an account of these kinds of identity conditions for persons (or tables, trees, bicycles, ships). Often philosophers will talk about candidates for being a single person (or table, tree, bicycle, ship) where the candidates are picked out or identified at different times. We talk about a fly before losing a body part, or before dying, and a fly after those changes, and argue about/try to figure out whether that’s still (numerically) the same fly. What makes this fly one and the same fly as the one we saw yesterday? What makes it a different fly from the one upstairs (which may be qualitatively very similar to it). To answer questions like that is to say what the fly’s identity conditions are.